Secondary recovery process using brine soluble terpolymers

ABSTRACT

Hydrocarbons are recovered from a subterranean hydrogen-bearing formation penetrated by an injection well and a production well by displacing hydrocarbons toward the production well using a drive fluid such as water thickened with a terpolymer of acrylamide, acrylonitrile and acrylic acid or a terpolymer of acrylamide, acrylonitrile and acrylic acid alkoxylated with ethylene oxide. If desired, the drive fluid may be saturated with carbon dioxide and/or natural gas at the injection pressure.

This is a divison of application Ser. No. 158,657, filed June 11, 1980.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for recovering hydrocarbons from asubterranean hydrocarbon-bearing formation penetrated by an injectionwell and a production well wherein a device fluid such as waterthickened with a terpolymer of acrylamide, acrylonitrile and acrylicacid or a terpolymer of acrylamide, acrylonitrile and acrylic acidalkoxylated with ethylene oxide or a mixture of ethylene oxide andpropylene oxide, is utilized to displace hydrocarbons in the formationtoward a production well.

2. Prior Art

The production of petroleum products is usually accomplished by drillinginto a hydrocarbon-bearing formation and utilizing one of the well-knownrecovery methods for the recovery of hydrocarbons. However, it isrecognized that these primary recovery techniques may recover only aminor portion of the petroleum products present in the formationparticularly when applied to reservoirs of viscous crudes. Even the useof improved recovery practices involving heating, miscible flooding,water flooding and steam processing may still leave up to 70-80 percentof the original hydrocarbons in place.

Thus, many large reserves of petroleum fluids from which only smallrecoveries have been realized by present commercial recovery methods,are yet to reach a potential recovery approaching their estimatedoil-in-place.

Water flooding is one of the more widely practiced secondary recoverymethods. A successful water flood may result in recovery of 30-50percent of the original hydrocarbons left in place. However, generallythe application of water flooding to many crudes results in much lowerrecoveries.

The newer development in recovery methods for heavy crudes is the use ofsteam injection which has been applied in several modifications,including the "push-pull" technique and through-put methods, and hasresulted in significant recoveries in some areas. Crude recovery of thisprocess is enhanced through the beneficial effects of the drasticviscosity reduction that accompanies an increase in temperature. Thisreduction in viscosity facilitates the production of hydrocarbons sinceit improves their mobility, i.e., it increases their ability to flow.

However, the application of these secondary recovery techniques todepleted formations may leave major quantities of oil-in-place, sincethe crude is tightly bound to the sand particles of the formation, thatis, the sorptive capacity of the sand for the crude is great. Inaddition, interfacial tension between the immiscible phases results inentrapping crude in the pores, thereby reducing recovery. Anotherdisadvantage is the tendency of the aqueous drive fluid to finger, sinceits viscosity is considerably less than that of the crude, therebyreducing the efficiency recovery. Another disadvantage is the tendencyof the aqueous drive fluid to finger, since its viscosity isconsiderably less than that of the crude, thereby reducing theefficiency of the processes. Another disadvantage is the tendency of theaqueous drive fluid to remove additional gas by diffusion from thein-place oil thus further reducing the already lowered formation oilvolume and increasing the viscosity of the oil.

There is a definite need in the art for a water flooding process inwhich the disadvantages discussed above are largely eliminated oravoided.

SUMMARY OF THE INVENTION

This invention relates to a process for recovering hydrocarbons from asubterranean, hydrocarbon-bearing formation penetrated by an injectionwell and a production well which comprises:

(a) injecting into the formation via an injection well a drive fluidcomprising brine having dissolved therein a small amount of a terpolymercomprising recurring units of: ##STR1## wherein x, y and z represent theweight percent of the respective units in the said terpolymer wherein Mis selected from the group consisting of hydrogen, sodium, potassium,ammonium or the radical

    --(CH.sub.2 CH.sub.2 O).sub.n H

wherein n is an integer of from 1 to 3 inclusive,

(b) forcing the said fluid through the formation and

(c) recovering hydrocarbons through the production well.

An additional embodiment of this invention relates to the driving fluidcompositions utilized in step (a) which may be saturated with carbondioxide and/or natural gas at the injection pressure, if desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE illustrates the results obtained in capillary displacementtests utilizing water alone and employing an aqueous solution having20,000 parts per million NaCl hardness plus 0.025 weight percent of abrine-soluble acrylamide-acrylonitrile-sodium acrylate terpolymer.

DETAILED DESCRIPTION OF THE INVENTION

Prior to practicing the process of this invention it is sometimesdesirable to open up a communication path through the formation by ahydraulic fracturing operation. Hydraulic fracturing is a well-knowntechnique for establishing a communication path between an injectionwell and a production well. Fracturing is usually accomplished byforcing a liquid such as water, oil or any other suitable hydrocarbonfraction into the formation at pressures which are sufficient to rupturethe formation and to open up channels therein. By use of this method itis possible to position the fracture at any desired vertical locationwith respect to the bottom of the oil-filled zone. It is not essentialthat the fracture planes be horizontally oriented, although it is, ofcourse, preferably that they be. After the fracture has beenestablished, and without diminishing the fracture pressure, a proppingagent may be injected into the fraction in order to prevent healing ofthe fracture which would destroy its usefulness for fluid flowcommunication purposes. Gravel, metal shot, glass beads, sand, etc. andmixtures thereof are generally employed as propping agents. When sand isutilized as the propping agent particles having a Tyler mesh size offrom about 8 to about 40 are preferred (i.e., from about 0.16 to about0.093 inches).

As previously pointed out the terpolymers employed in this inventioncomprise recurring units of: ##STR2## wherein x, y and z represent theweight percent of the respective units and wherein the sum of x, y and zis 100, wherein M is selected from the group consisting of hydrogen,sodium, potassium, ammonium or the radical

    --(CH.sub.2 CH.sub.2 O).sub.n H

wherein n is an integer of from 1 to 3 inclusive. In the terpolymersemployed in this invention x will vary from about 50 to about 60 weightpercent, y will vary from about 20 to about 30 weight percent and z willrange from about 10 to about 20 weight percent. Generally, the numberaverage molecular weight of the terpolymers will range from about 10,000to about 2,000,000 or more. The terpolymers of this invention areinsoluble in water but are soluble in brine having a total hardness ofabout 2×10³ ppm to about 2×10⁵ ppm.

Terpolymers of acrylamide, acrylonitrile and acrylic acid or the sodium,potassium or ammonium salts thereof which are water soluble are knownand the preparation of such terpolymers is described in the U.S. Pat.No. 3,039,529. In this same patent the use of the water solubleterpolymers in a secondary recovery process for petroleum is disclosed.

Surprisingly it has been found that the terpolymers of this invention asdescribed above although not soluble in water are soluble in brine andare therefore highly useful in drive fluids where brine is utilized asthe aqueous fluid.

The brine-soluble terpolymers of this invention can be prepared by anumber of vinyl polymerization routes which are well known in the art.These terpolymers can also be conveniently prepared by the process setout in the U.S. Pat. No. 3,039,529 which is incorporated herein in itsentirety.

Where one of the components of the brine-soluble terpolymers of thisprocess is alkoxylated acrylic acid such alkoxylated monomers can beconveniently prepared by methods well known in the art. For example, anaqueous solution of acrylic acid comprising about 10 to about 30 weightpercent or more of the acid in water along with about 0.5 weight percentor more of powdered potassium hydroxide or sodium hydroxide is chargedto an autoclave and the autoclave and contents heated to a temperatureof about 125° to about 200° after which the required weight of ethyleneoxide or a mixture of ethylene oxide and propylene oxide is pressuredwith nitrogen into the autoclave over a period of 1 to about 3 hours ormore following which the autoclave is allowed to cool to roomtemperature and then vented. The reaction product remaining after beingstripped to remove volatile materials yields the water-soluble,alkoxylated acrylic acid product. Alternatively, the alkoxylated acrylicacid component can be formed by reacting a polyoxyalkylene glycol ofsuitable molecular weight with acrylic acid in the presence of, forexample, potassium hydroxide.

A number of other method are set out in the art for conducting suchalkoxylation reactions including those described in U.S. Pat. Nos.2,213,477, 2,233,381; 2,313,142; 3,879,475; 2,174,761; 2,425,845 and3,062,747.

The following example illustrates the preparation of the brine-solubleacrylamide-acrylic acid-acrylonitrile terpolymers useful in thisinvention.

PREPARATION OF POLYMER A

    ______________________________________                                        Materials:                                                                             (1)   Acrylamide          60 gm.                                              (2)   Acrylonitrile       20 gm.                                              (3)   Acrylic Acid        20 gm.                                              (4)   Sodium Chloride     25 gm.                                              (5)   Potassium Persulfate                                                                              0.08 gm.                                            (6)   Ethylene Diaminetetraacetic                                                                       0.05 gm.                                                  Acid Disodium Salt (EDTAA)                                              (7)   Sodium Carbonate    30 gm.                                              (8)   Distilled Water     500 ml.                                    ______________________________________                                    

A 1 liter resin kettle was charged with the above monomers, sodiumchloride, sodium carbonate and EDTAA, dissolved in distilled water andpurged with pre-purified nitrogen while heating to 53° C. When thetemperature reached 53° C. (after 30 minutes), the potassium persulfateinitiator was added and the polymerization reaction was allowed toproceed over a period of 6 hours. The resultant viscous rubbery mass wasbroken up into small chunks, dropped into 1500 ml. of methanol and themixture stirred for 1 hour. The solid polymer was then placed in aWaring Blender with some methanol and ground into small granules,filtered on a Buchner funnel and dried in a vacuum oven to constantweight at 60° C. (<1 mm) to yield 130.8 gm. of recovered polymericproduct (Polymer A).

Polymer A was evaluated and the test results are reported below:

    ______________________________________                                                       Polymer A.sup.a                                                                        Polymer B.sup.b                                       ______________________________________                                        Viscosity at 300 ppm in                                                                        Insoluble  7.36                                              H.sub.2 O                                                                     Screen Viscometer Factor*                                                                      Insoluble  8.0                                               Viscosity at 300 ppm in                                                                        1.32       1.36                                              2% NaCl                                                                       Screen Viscometer Factor                                                                       6.0        6.0                                               % Polymer Loss in 24 Hr.                                                                       16%        19%                                               Adsorption                                                                    (Pump Test in Berea                                                           Sandstone Core)                                                               ______________________________________                                         .sup.a Polymer composed of acrylamide/acrylonitrile/sodium acrylate in        50/20/25 wt. ratio was insoluble in both distilled water and 2% NaCl          whereas Polymer A having a 60/20/20 wt. ratio is soluble in brine and         insoluble in water.                                                           .sup.b Polymer B is a commercially available partially hydrolyzed             polyacrylamide sold under the tradename "Pusher 700" by the Dow Chemical      Company.                                                                      ##STR3##                                                                 

In order to study the displacement performance of a brine solution ofPolymer A (Polymer concentration--0.025 weight percent), laboratorydisplacement tests were conducted on a radial core obtained from a Bereasand field. The core which was approximately 4 inches in outsidediameter had an inside bore diameter of 0.5 inches and was approximately4 inches long. The permeability of the core was about 12,480millidarcies. A similar test was conducted to determine the response toa conventional water flood conducted to break-through of the injectedwater in order to obtain a comparison of the results realized usingpolymer thickened brine and water alone. In each test the clean,evacuated radial core was filled with simulated connate water which wasthen displaced by stock tank oil to establish initial oil saturation.Both floods were performed at the same constant displacement rate, andthe results obtained are given in the Figure. It can be seen that themaximum recovery which occured at one pore volume of injected fluidusing a conventional water flood was in the range of 75 percent, whereasapproximately 90 percent recovery was achieved utilizing the terpolymercomposition of this invention in brine having an NaCl concentration of20,000 ppm.

In the secondary recovery process of this invention, generally theaqueous drive fluid will contain from about 0.01 to about 5.0 weightpercent or more of the brine-soluble, acrylamide-acrylonitrile-acrylicacid or alkoxylated acrylic acid terpolymer. Optionally, the aqueousdrive fluid may be saturated with carbon dioxide and/or natural gas atthe injection pressure which generally will be from about 300 to about3000 psig or more.

If desired, the aqueous drive fluid having dissolved therein theabove-described polymeric thickening agent may be made alkaline byaddition of an alkaline agent. The advantageous results achieved withthe aqueous alkaline medium used in the process of this invention arebelieved to be derived from the wettability improving characteristics ofthe alkaline agent.

Useful alkaline agents include compounds selected from the groupconsisting of alkali metal hydroxides, alkaline earth metal hydroxides,and the basic salts of the alkali metal or alkaline earth metals whichare capable of hydrolyzing in an aqueous medium to give an alkalinesolution. The concentration of the alkaline agent employed in the drivefluid is generally from about 0.005 to about 0.3 weight percent. Also,alkaline materials such as sodium hypochlorite are highly effective asalkaline agents. Examples of these especially useful alkaline agentsinclude sodium hydroxide, potassium hydroxide, lithium hydroxide,ammonium hydroxide, sodium hypochlorite, potassium hypochlorite, sodiumcarbonate and potassium carbonate.

A wide variety of surfactants such as linear alkylaryl sulfonates, alkylpolyethoxylated sulfates, etc. may also be included as a part of theaqueous drive fluid composition. Generally about 0.001 to about 1.0 ormore weight percent of the surfactant will be included in the drivefluid.

This invention is best understood by reference to the following examplewhich is offered only as an illustrative embodiment of this inventionand is not intended to be limitative.

EXAMPLE I

In a field in which the primary production has already been exhausted,an injection well is completed in the hydrocarbon-bearing formation andperforations are formed between the interval of 5680-5695 feet. Aproduction well is drilled approximately 440 feet distance from theinjection well, and perforations are similarly made in the samehydrocarbon-bearing formation at 5685-6000 feet.

The hydrocarbon-bearing formation in both the injection well and theproduction well is hydraulically fractured using conventionaltechniques, and a gravel-sand mixture is injected into the fracture tohold it open and prevent healing of the fracture.

In the next step brine saturated with carbon dioxide at 1200 psig at atemperature of 70° F. to which there has been added about 0.10 weightpercent sodium hydroxide and containing dissolved therein 0.40 weightpercent of an acrylamide-acrylonitrile-sodium acrylate terpolymer havinga number average molecular weight of about 125,000 is injected via theinjection well into the formation at a pressure of 1200 psig and at therate of 0.95 barrel per minute. In the acrylamide-acrylontrile-sodiumacrylate terpolymer the weight percent of the acrylamide units is about58, the weight percent of the acrylonitrile units is 24 with the balancebeing sodium acrylate units. Injection of the drive fluid is continuedat the rate of about 1 barrel per minute and at the end of 65 days therate of production of oil is substantially greater than with waterinjection alone.

What is claimed is:
 1. A brine-soluble terpolymer comprising recurringunits of: ##STR4## wherein x, y and z represent the weight percent ofthe respective units in the terpolymer and x ranges from about 50 toabout 60 weight percent, y ranges from about 20 to about 30 weightpercent and z ranges from about 10 to about 20 weight percent andwherein M is the radical

    --(CH.sub.2 CH.sub.2 O).sub.n H

wherein n is an integer of from 1 to 3 inclusive.
 2. The terpolymer ofclaim 1 wherein the number average molecular weight ranges from about10,000 to about 2,000,000.